Setting Priority for a certain Site/program?

Hi there. i do quite a lot of online gaming thru an online VPN client called Garena. i'm connected to the net thru a WRT54GL runnin Tomato 1.22 with Victek's mod. problem is, when my other friends are using a direct download, it screws up my gaming experience with spikes and lags, i've tried manually adding the ports to the QOS and Vegas is also enabled, but is there any way to prioritize upload/download for myself? Thanks loads